Rheostat



F 1943- G. L. M FARL'AND, JR 2,312,110

. RHEOSTAT Filed Jan. 28, 1942 .Fiip i Inventom Gebrge L. Mc Far-land JTT,

Hi Attovn ey.

Patented Feb. 23, 1943 RHEOSTAT George L. McFarland, Jr., Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application January 28, 1942, Serial No. 428,567

6 Claims.

My invention relates to rheostats, more particularly variable or adjustable rheostats, such as used in the control of electrical apparatus, and has for its object a simple, low cost and rugged rheostat and, furthermore, a rheostat made up of a plurality of electric resistors which are replaceable in case of damage and whose electric resistance may be graduated as desired.

In carrying out my invention in one form, I utilize as resistors or resistance units a plurality of relatively short resistance heating units of the sealed metal encased type which I arrange radially and clamp together by means of plates provided with apertures for ventilating the inner ends of the resistance units. I also provide an insulating support in the center of these radially arranged units on which are a plurality of contacts connected to the terminals of the units and a contact arm engaging these contacts by means of which the amount of the resistance in the electric circuit can be varied.

For a more complete understanding of my invention, reference should be had to the accompanying drawing in which Fig. 1 is a plan view of a dial type rheostat embodying my invention; Fi 2 is a view mainly in vertical section of the device shown in Fig. 1; while Figs. 3 and 4 are fragmentary views showing details of construction.

Referrin to the drawing, in one form of my invention I provide a plurality of resistance units or resistors III consisting of metal encased heating units. Seventeen of these resistors II) are shown arranged radially so as to leave a circular opening in the center of these radially arranged I resistors. In this central opening is a support II made of a suitable electrically insulating material such as a heat resisting compound, this support being circular in shape and somewhat smaller than the central opening defined by the resistors so as to be spaced from the inner ends of the resistors.

On the periphery of the support I I arranged in equally spaced relation with each other are a plurality of electric contacts I2 having their upper surfaces, as seen in Fig. 2, substantially flush with each other and lying in a plane surface thereby to be engaged by a brush I3 on the end of a contact arm I4 secured to an operating knob or handle I5. This handle is mounted on a pivot pin l6 pivotally mounted centrally on the support I I so that by turning the handle the contact brush l 3 is moved over the contacts I2.

The terminalleads II from each of the resistors ID are suitably connected electrically to the contacts I 2. As shown, each resistor has four terminals I1 extending from its inner end, which terminals are connected to four contacts with the adjacent terminals of two adjacent resistors connected to the same contact whereby the whole series of seventeen resistors is connected in a closed electric circuit. At one side, as shown, the lower side, a substantial space is left between the two adjacent resistors into which extends a projection Ila on the insulating support I I. The two end contacts I2 are connected by conductors I8 and I9 to electric terminals 28 and 2I on this extension II a. Also, a third terminal 22 is connected by conductor 23 to the contact. arm I4. This latter connection is preferably eiiected by connecting the conductor 23 to a circular plate 24 secured so as to establish a circuit between'the plate 24 and the contact I2 on which the brush I3 bears.

The plurality of resistors and the support II are clamped firmly together in the relation shown by means of upper and lower metallic clamping and supporting plates 26 and 21. These plates are circular in form and of the same circular size, and each has a series of radial indentations 28 extending respectively downward and upward between the pairs of resistors. Also, the indentations 28 and 29 are slightly wider at their'outer ends so as to fit closely between the pairs of resistors. These plates are clamped together by a series of bolts 30 extending through the indentations 28 and 29 between each pair of resistors, the

two plates having their centers coincident withv the center line of the pin I6.

At its center, the lower plate 21 is bent upward somewhat to form a raised support 3| for the insulating support II which is secured thereto by bolts 32. As shown, the support II is raised far enough so that its lower edge is slightly above the resistor terminals IT, as seen in Fig. 2, so that terminals can be easily secured to the bottoms of the contacts I2. As shown in Fig. 2, each contact preferably is a strip of metal arranged radially with respect to the pin IIS and having end portions extending downward through the support and bent over toward each other on the bottom of the support. A series of notches around the periphery of the support II is provided for the outer lengths of the contacts I2.

The center of the upper plate 26 is raised somewhat to form a cover portion 33 over the contact arm Id, the cover being provided with an aperture 34 through which the knob I5 extends, this aperture being considerably larger than the knob to provide ventilation. Additional ventilating apertures 35 are provided around the side supporting wall of the raised portion 33. Likewise, the ventilating apertures 36 are provided in the supporting wall of the bottom support 3 l.

The resistors ID are the type of heating unit in which th resistance conductor 31 is embedded in refractory insulating material 38, such as magnesium oxide, which is suitably compacted as by reducing the cross section of the metal sheath 3101. containing the insulating material. The heating unit may be somewhat similar in construction to that described and claimed in United States Patent No. 1,904,594, issued to Charles C. Abbott on April 18, 1933. Although I have shown four terminals ll coming out of the end of each heating unit and extending through a suitable insulator 39 at the end of the sheath, any suitable number can be used. As shown, the resistance conductor 31 is passed back and forth in a plurality of zigzag lengths from one end of the heating unit to the other, the ends of the resistance conductor being connected to the two outside terminals and each loop being. connected electrically to an inside terminal.

In use, the entire resistance may be connected in the electric circuit by electrical connections to the terminals 28 and 21. Th central terminal 22 will be electrically connected to one of the outside terminals so that the resistance between this outside terminal and the arm I4 is short-circuited. Thusby turning the knob t5, a desired amount of the resistan'cecan be cut in or out thereby to vary the amount included in the, electric circuit.

Moreover; by using resistor units each having different electrical resistances and by suitably arranging them, any desired graduation of resistance or resistance gradient can be obtained. Another advantage is that any resistor unit which may become damaged can be readily replaced without replacement of theentire rheostat.

Moreover, the rheostat is adapted to withstand shocks and vibrations without damage. This is particularly true because of the mounting of the resistance conductor 3! in compacted insulating material 38 so that it is held securely against: movement. Also, this rheostat can be operat'ed at high temperatures because the resistance conductors are sealed from contact with the air in the insulating material, and as a result a larger current can be passed through the resistance conductor for a given temperature rise.

Furthermore, this rheostat is especially adapted for high heat dissipation. qualities by reason of the freedom of air movement over large sur faces of the resistor; Thus the outer ends of the radially extending resistors I are spaced apart so that: cooling air can circulate freely around the exposed outer ends projecting from between the clamping plates 28 and 29. Furthermore the clamping plates, as shown in Fig. 4, are spaced apart at points between the resistors to provide a duct for the circulation of cooling air radially between the resistors.

Also, the apertures 35 and 36 provide for the free'circulation of air through the center of the rheostat when the rheostat is mounted in a horizontal position. Moreover, with the rheostat in a horizontal position, an effective radial circulation of cooling. air between the resistors will be set up. When the rheostat is mounted in a vertical position, the circulation of air through the central portion may not be as vigorous as compared with the horizontal position, but on the othenhand, thecirculation of air radially between the resistors will be more pronounced with the rheostat in the vertical position except possibly for those resistors which are in nearly a horizontal position.

The two clamping plates 28 and 29, of course, are in intimate thermal relation with the resistors by reason of the fact that they are clamped on the resistors. The plates, therefore, receive heat freely by conduction directly from the resistors and dissipate this heat to the air circulating around the rheostat.

While I have shown a particular embodiment of my invention, it will be understood, of course, that I do not wish to be limited thereto, since many modifications may be made and I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A rheostat comprising a plurality of metal sheathed electric resistance units, upper and lower metal clamping plates between which saidresistance units are mounted in positions to extend radially with respect to a common center, means securing said plates together thereby to clamp said resistance units between said plates in good heat conducting relation with said plates, said resistance units being arranged to provide a space at their inner ends, and selective means in said space secured to one of said plates, for varying the number of said units connected in an electric circuit.

2. A rheostat comprising a plurality of electric resistance units, upper and lower plates between i which said resistance units are secured in positions to extend radially with respect to a common center, means extending between said resistance units for pulling said plates together thereby to clamp said resistance units between said plates in good heat conducting relation with said plates, said resistance units being arranged to provide a central space at their inner ends, and selective means in said central space mounted on one of said plates for varying the number of said units in an electric circuit, said plates being spaced apart between said resistance units and being provided with apertures communicating with said central space thereby to provide for the circulation of cooling air through said central space and from said central space radially outward between said plates and said resistance units.

3. A rheostat comprising a plurality of elongated electric resistance units each provided with a plurality of electric terminals extending from. one end, upper and lower metal clamping plates between which said resistance units are clamped in positions to extend radially with respect to, a predetermined center, said resistance units being arranged to provide a space at their inner ends and with said terminal ends constituting said inner ends, a central raised portion on said lower clamping plate extending into said space, a support of electrically insulating material mounted on said raised portion, a plurality of electric contacts secured to said insulating support, a contact arm mounted on said insulating support and moving over said contacts, electric connections between said terminals of said resistance units and said contacts whereby said resistance units are connected together in series with each other, and a central raised portion on said upper clamping plate enclosing said contact arm.

4. A rheostat comprising a plurality of elongated electric resistance units each provided with a plurality of electric terminals extending from one end, each of said resistance units being provided with a metallic sheath and with a resistance conductor embedded in electrically insulating material in said sheath, upper and lower metal clamping plates between which the metallic sheaths of said resistance units are mounted thereby to hold said resistance units spaced apart and in positions to extend radially with respect to a predetermined center, means extending between said units for pulling said plates together thereby to clamp said resistance units tightly between said plates in good heat conducting relation with said plates, said resistance units being arranged to provide a space at their inner ends and with said terminal ends constituting said inner ends, and selective means mounted on one of said clamping plates in said space, for varying the number of said units connected in an electric circuit.

5. A rheostat comprising a plurality of elongated electric resistance units each provided with a plurality of electric terminals extending from one end, each of said resistance units being provided with a metallic sheath and with a resistance conductor embedded in electrically insulating material in said sheath, upper and lower circular metal clamping plates between which the metallic sheaths of said resistance units are clamped thereby to hold said resistance units in positions to extend radially with respect to the center of said plates, a circular support of electrically insulating material mounted, on said lower clamping plate, a plurality of electric contacts secured to the periphery of said insulating support, a contact arm pivotally secured to said insulating support and moving over said contacts, electric connections between said terminals of said resistance units and said contacts whereby said resistance units are connected together in series with each other, and ventilating apertures in the central portions of said plates.

6. A rheostat comprising a plurality of elongated electric resistance units each provided with a plurality of electric terminals extending from one end, each of said resistance units being provided with a metallic sheath and with a resistance conductor embedded in electrically insulating material in said sheath, upper and lower circular metal clamping plates between which the metallic sheaths of said resistance units are clamped thereby to hold said resistance units in positions to extend radially with respect to the center of said plates, said resistance units being arranged to provide a circular space at their inner ends and with said terminal ends constituting said inner ends, a central raised portion on said lower clamping plate, a circular support of electrically insulating material mounted on said raised portion, a plurality of electric contacts secured to the periphery of said insulating support, a contact arm pivotally secured to the center of said insulating support and having a contact brush moving over said contacts, electric connections between said terminals of said resistance units and said contacts whereby said resistance units are connected together in series with each other, a central raised portion on said upper clamping plate enclosing said contact arm, and ventilating apertures in the peripheral supporting walls of said raised portions.

GEORGE L. MCFARLAND. J R. 

